Why is the value measured when the voltage measurement terminal is floating?

“When the power analyzer and power meter test the voltage, sometimes it is found that the test voltage has not been connected, and the instrument displays a certain value. In fact, this is not an instrument failure. Here we will reveal the secret together.

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When the power analyzer and power meter test the voltage, sometimes it is found that the test voltage has not been connected, and the instrument displays a certain value. In fact, this is not an instrument failure. Here we will reveal the secret together.

The principle of electromagnetic induction

Do you still remember the physics knowledge we learned in high school, the principle of electromagnetic induction. I believe that many people must be very painful when learning the principle of electromagnetic induction and the left-right rule. Let’s review it first. The law of electromagnetic induction is also called Faraday’s law of electromagnetic induction. The phenomenon of electromagnetic induction refers to the phenomenon of induced electromotive force due to the change of magnetic flux. For example, when a part of the conductor of a closed circuit moves to cut the magnetic field line in the magnetic field, a current will be generated in the conductor. The resulting current is called induced current, and the resulting electromotive force (voltage) is called induced electromotive force. Therefore, the movement of the conductor in the magnetic field will generate an induced current and form an induced electromotive force (voltage). Similarly, a stationary conductor will also generate an induced current in a changing magnetic field to form an induced electromotive force (voltage). This is the principle of electromagnetic field change.

The electromagnetic environment in the air

Nowadays, our life is inseparable from electricity. Household appliances, battery cars, electric cars, broadcasting, motors, etc., so there will be various electromagnetic fields in the air, all of which will affect the measuring instruments. The electromagnetic field, we call it electromagnetic noise.

The general electromagnetic noise will not affect the test measurement. The frequency range of various electromagnetic noises is different, and it will continue to attenuate in the air, so it can usually be ignored. However, due to the existence of electromagnetic noise, it will affect some test instruments with relatively high bandwidth and precision. For example, products such as power meters and power analyzers have high precision and large bandwidth. If they are improperly operated in small ranges, they are easily affected by electromagnetic interference and cause misunderstandings. For example, when the voltage terminal of the instrument is suspended and the range is set to a small value, the instrument will Display a certain voltage value, which is a typical result affected by electromagnetic noise.

3. How to avoid the influence of noise

First of all, we have to explain a problem. The state where the input of the voltage channel is zero is not actually an open circuit state, but a state of input short circuit (connecting the high and low terminals with a short line). The input impedance of the voltage channel measurement is extremely high, and the space electromagnetic noise will be picked up in the open-circuit state, and the voltage measurement value will not be zero. Therefore, if we find that the voltage is not zero when the instrument is suspended, we can take a short wire, short the high and low ends of the voltage port of the instrument, and then check whether the value is zero. At this time, the voltage channel is in the zero state, and the display is also is zero. In addition to short-circuiting, we also pay attention to some key settings of the instrument, such as whether the PT ratio is set correctly, whether the PT switches normally, whether the voltage range is appropriate, and whether the power supply and grounding of the instrument are good to ensure the final measurement is accurate.

There is not only the atmosphere in the air, but also many electromagnetic fields that we can’t see. Although we can’t see it, it still affects our test and measurement. I hope this article can solve some of your questions in the measurement.